Structural member for a motor vehicle

ABSTRACT

A structural member of a motor vehicle, e.g., a “B” pillar, comprises a reinforcing member having an elongate core of ultra high strength steel and a casing of aluminum alloy, the casing having a constant section profile with a closed section cavity. The core is encased within the casing and the core and the casing bonded together by co-drawing the core and the casing through a die. The casing has a pair of spaced flanges which connect the reinforcing member to aluminum alloy pressings to form the structural member. The reinforcing member has considerable yield strength from the use of the ultra high strength steel in the core while the flanges, being of aluminum alloy, are readily joined by self-piercing rivets.

FIELD OF THE INVENTION

This invention relates to motor vehicles and in particular to theproduction of a motor vehicle having low weight and a high resistance todeformation in the event of a collision.

BACKGROUND OF THE INVENTION

It is well known that in order to improve fuel efficiency and emissionsit is desirable to produce a lightweight motor vehicle. In order toachieve this goal, several vehicle manufacturers have proposed vehiclesin which a substantial proportion of the body structure or bodyshell isconstructed from a lightweight material such as an aluminum alloy. Anexample of such a vehicle construction is shown in U.S. Pat. No.6,099,071.

Some sections of such a bodyshell act as beams for which a substantialyield strength is a principal requirement. For a bodyshell assembledfrom aluminum pressings and extrusions, the yield strength of thealuminum dictates the thickness of material required in these places.This can result in the design being bulky, heavy and expensive comparedto a design using high strength steel, thereby eliminating much of theanticipated weight saving from the use of aluminum.

One way of optimizing the use of aluminum is to integrate a steelreinforcement into the structure. However, many existing methods ofjoining steel and aluminum such as brazing, adhesive bonding, bolting,or riveting have disadvantages in motor vehicle production, particularlyfor mass-produced passenger cars. In particular, brazing providesinadequate joint strength. Adhesive bonding requires supplementarymechanical fixings such as riveting or bolting to avoid peel failure andto fix the geometry of an assembly prior to curing. Bolting is timeconsuming and difficult to accommodate where space on a component islimited, e.g., in the posts, or pillars of the upper structure of abodyshell. Blind riveting is more space efficient than bolting but canbe even more time consuming.

One quick, space efficient and repeatable method for fastening togetherbodyshell components in a mass production environment is the use of aself-piercing riveting process such as described in U.S. Pat. No.5,752,305, hereby incorporated by reference.

Hence it is desirable to be able to join a high yield strength steelreinforcement to an aluminum structure using self-piercing rivets sincethis can be done in a cost effective manner and produces a joint withthe required strength. For steels having a low to moderate yieldstrength (e.g. up to 440 MPa) such joining is possible by directlyfastening the steel to aluminum using self pierce riveting. However,such steels do not have a sufficiently high yield strength to producethe required combination of low weight and resistance to bending. Whensteel having a sufficiently high yield strength (e.g. 950 MPa) is used,then self-piercing rivets cannot be used because the steel is too hardto yield during the riveting process.

The reinforcement of aluminum profiles with steel is known, e.g., fromBE511181 where an aluminum glazing profile is shown and fromJP57-001516A where a trolley wire is described as being made byextruding an aluminum coat over a steel core. However, in neither caseis the profile suitable for a structural member of a motor vehicle wherehigh strength has to be combined with the ability to readily join thestructural member to other components of the bodyshell. While in U.S.Pat. No. 5,941,597, a steel reinforcement of an aluminum extrusion hasbeen proposed for such a structural member, this is simply fitted insidethe extruded section to increase stiffness.

It is thus an object of this invention to provide a structural memberfor a motor vehicle bodyshell which includes a reinforcing member of lowweight and a high resistance to bending and can be easily joined toother components of the bodyshell.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided astructural member for a motor vehicle, the structural member comprisingat least one panel member and a reinforcing member, the reinforcingmember comprising an elongate core of a relatively high yield strengthmetal and a casing of a relatively low yield strength metal having aconstant section profile with an internal wall defining a closed sectioncavity, the core being encased within the casing and the core and thecasing being bound together by co-drawing the core and the casingthrough a die, the casing having a pair of spaced flanges connecting thereinforcing member to said at least one panel member.

At least one of the casing and the internal wall of the core may becoated with an adhesive before co-drawing to increase bonding betweenthe core and the casing.

The core may be made from ultra high strength steel, preferably withyield strength of at least 500 MPa. More advantageously, the yieldstrength is at least 900 MPa

The casing may be made from lightweight non-ferrous material, preferablyan aluminum alloy.

The core may be profiled flat strip, non-planar in cross-section andhaving two longitudinally extending edges, in which case the materialforming the longitudinal edges may be bent back on itself to formdouble-thickness end sections to reduce the stress concentration effectof the core on the casing. Alternatively, the core may be tubular.

Preferably, the casing is joined to at least one panel member byself-piercing rivets.

In a second aspect of the invention, there is provided a method ofmanufacturing a body structure for a motor vehicle including the stepsof producing a number of reinforcing members by providing an elongatecore of a relatively high yield strength metal, a casing of a relativelylow yield strength metal and having a constant section profile with apair of spaced flanges and an internal wall defining a closed sectioncavity, placing the core within the cavity and co-drawing the core andthe casing through a die to bind the casing to the core, producing anumber of panel members each having a pair of spaced flangescorresponding to the spaced flanges on the reinforcing member,connecting the reinforcing members and the panel members at the spacedflanges to form structural members, providing a number of body panelsand attaching the structural members and the body panels together in apredetermined arrangement to produce the body structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, of which:

FIG. 1 is a left hand side view of a motor vehicle having a structuralmember according to the invention;

FIG. 2 is a cross section through a “B” post, or “B” pillar, formingpart of the body structure of the vehicle shown in FIG. 1 andincorporating one example of a reinforcing member.

FIG. 3 is a side view illustrating how a reinforcing member shown inFIG. 2 is adapted for use in the “B” post shown in FIG. 1;

FIG. 4 is a cross section similar to FIG. 2 showing a first modificationto the reinforcing member shown in FIG. 2;

FIG. 5 is a cross section similar to FIG. 2 showing a secondmodification to the reinforcing member shown in FIG. 2;

FIG. 6 is a cross section through the “B” post forming part of the bodystructure of the motor vehicle shown in FIG. 1 showing a second exampleof a reinforcing member;

FIG. 7 is a cross section similar to FIG. 6 showing a first modificationto the reinforcing member shown in FIG. 6;

FIG. 8 is a cross section similar to FIG. 6 showing a secondmodification to the reinforcing member shown in FIG. 6; and

FIG. 9 is a cross section through an alternative form of “B” post foruse in the body structure of the motor vehicle shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1 there is shown a motor vehicle 10 having a bodystructure 1 comprised of a number of aluminum panels fastened to anumber of underlying aluminum structural members (not visible on FIG. 1)and aluminum side structural members including an “A” post 2, a “B” post3, a “C” post 4, a “D” post 5 and a “E” post 6, the side structuralmembers being used to connect a roof 9 to a lower body structure. Thevehicle 10 also includes a number of doors including a front door 7 anda rear door 8.

Referring further to FIG. 2, the “B” post 3 is formed from a firstelongate panel member in the form of an aluminum pressing or stamping 11and a second elongate panel member in the form of another aluminumpressing or stamping 12. The first pressing 11 has a first flange 13extending along one edge and a second flange 15 extending along anopposite edge. Similarly the second pressing 12 has a first flange 14extending along one edge and a second flange 16 extending along anopposite edge. The first and second pressings 11 and 12 when assembledtogether form an internal volume 19 in which is fastened a reinforcingmember 20.

The reinforcing member 20 comprises a steel core 21 encased in analuminum casing 22. The core 21 is formed of profiled flat strip, beingnon-planar in its transverse cross-section so as to improve itsresistance to bending. The casing 22 has a body portion 27 in which thecore 21 is located and two longitudinally extending flanges 23, 24 ofuniform width which extend along the length of the body portion 27.

The reinforcing member 20 can be manufactured in the following manner.The steel core 21 is formed into a constant section profile having thedesired shape, e.g. by rolling from flat strip. The aluminum casing 22is then extruded to form a constant section profile with an internalwall defining a closed section cavity 29. The shape of the cavity 29within the casing 22 is designed to match the geometry of the steel core21 and is sized so that the steel core 21 can be loosely fitted into thecavity 29. The casing 22 is then co-drawn with the steel core 21 througha die to bind the casing 22 to the core 21 in a manner substantially asdescribed in JP57-001516A, hereby incorporated by reference. Theresidual pressure of the casing 22 on the core 21 may well be sufficientto bind the casing 22 to the core 21 but for additional bonding anadhesive may be introduced between the core and the casing. If adhesiveis applied, it can be applied either to only the core 21, to only theinternal wall defining the cavity 29 in the casing 22, or to bothsurfaces. If adhesive is used, then this can be cured and the casing 22can be heat treated at the same time. After drawing, an elongate memberor beam is produced which can be left straight and cut to length to formthe reinforcing member 20. Alternatively, the elongate member or beamcan be cut to length and then be bent prior to heat treatment to form anappropriately shaped reinforcing member.

The reinforcing member 20 is fastened to the two pressings 11, 12 byplacing the reinforcing member 20 onto the first pressing 11 with itsflanges 23, 24 resting on and aligned with the first and second flanges13, 15 of the first pressing 11. The first and second flanges 13, 15 ofthe first pressings 11 and the flanges 23, 24 of the reinforcing member20 are then tacked together (i.e. joined at well-spaced locations) usingself-piercing rivets, e.g. as described in U.S. Pat. No. 5,752,305. Therivets only have to pass through aluminum and so relatively low forcesare required for this process. The second pressing 12 is then placed ontop of the reinforcing member 20 so that its first and second flanges14, 16 lie on top of and are aligned with the flanges 23, 24 of thereinforcing member 20. The first and second flanges 13, 15 and 14, 16,of the two pressings 11, 12 and the flanges 23, 24 of the reinforcingmember 20, are then secured together using self-piercing rivets asreferred to above but at closer locations.

Alternatively, the reinforcing member 20 is fastened to the twopressings 11, 12 by placing the reinforcing member 20 onto one of thefirst and second pressings 11 or 12 with its flanges 23, 24 resting onand aligned with the first and second flanges 13, 15 or 14, 16 of thefirst or second pressing. The other pressing 12 or 11 is then placed ontop of the reinforcing member 20 so that its first and second flanges14, 16 or 13, 15 lie on top of and are aligned with the flanges 23, 24of the reinforcing member 20. The first and second flanges 13, 15 and14, 16 of the two pressings 11, 12 and the flanges 23, 24 of thereinforcing member 20, are then secured together using self-piercingrivets as described above.

The completed structural member 3 can then be assembled into the bodystructure 1. Any panels or other members forming the body structure thatmate with the structural member 3 are designed to match it so thatflanges or designated matching joining areas can be brought into contactwith the flanges of the structural member 3 and be secured to it,preferably by self-piercing rivets. However, it will be appreciated thatbecause no steel is present in the flanges of the structural member 3,the structural member could be welded or bonded in position, e.g., asdescribed in EP0127343.

Alternatively, the reinforcing member 20 may be fastened to existingpanels of the body structure 1, the body panels that mate with thereinforcing member having flanges that correspond to the flanges of thereinforcing member to form a structural member which is integral withthe body structure.

After co-drawing, the width of the flanges 23, 24 are of the maximumwidth required to produce the desired flange shape. Prior to assembly ofthe reinforcing member 20 into the two pressings 11, 12, the flanges 23,24 can be trimmed so that their width is varied along their respectivelengths so that the reinforcing member 20 conforms to the shape requiredof the structural member 3. This is described in FIG. 3. The flanges 23,24 may be machined to produce the required shape or may be sheared orcut by laser. Alternatively, if required, the flanges 13, 14, 15, 16 ofthe pressings 11, 12 may also have the same maximum width when thepressings are assembled and riveted to the reinforcing member 20 so thatall the flanges 13, 14, 15, 16 & 23, 24 are trimmed to the requiredshape in the same operation. After the trimming operation thereinforcing member 20 may be bent to fit the other components asrequired. In a further variation, the reinforcing member 20 may be bentfirst and the trimming operation performed subsequently.

In a first alternative embodiment shown in FIG. 4, parts which are thesame as those shown in FIG. 2 carry the same reference while those whichare similar carry the same reference with the addition of 100. Thereinforcing member 120 differs from that described with reference toFIG. 2 in that the casing 122 is formed with transition portions 125,126 that change the stress gradient between the flanges 123, 124 and thebody portion 127 when a transverse bending load is applied to thestructural member 3. The reinforcing member 120 is manufactured exactlyas before and so will not be described again in detail.

In a second alternative embodiment shown in FIG. 5, parts which are thesame as those shown in FIG. 2 carry the same reference while those whichare similar carry the same reference with the addition of 200. Thereinforcing member 220 differs from that described with reference toFIG. 2 in that the casing 222 is formed with transition portions 225,226 similar to the transition portions 125, 126 in FIG. 4 to change thestress gradient between the flanges 223, 224 and the body portion 227when a transverse bending load is applied. Additionally, in order tominimize the stress raising effect of the steel core 221 on the casing222 when subject to a bending load, the steel core 221 has end sections231, 232 which are bent back upon themselves to form a double-thickness.This bending back increases the effective radius at the end of the steelcore 221 thereby reducing its stress raising effect on the casing 222.The reinforcing member 220 is manufactured exactly as before and so willnot be described again in detail.

FIG. 6 shows another embodiment of a reinforcing member 320 which isintended to be a direct replacement for the reinforcing members 20, 120,220 described with reference to FIGS. 2 to 5. Nevertheless, there aremany parts of the structural member 3 which remain substantially thesame as described in relation to FIGS. 2 to 5 so carry the samereference while those which are similar carry the same reference as inFIG. 2 with the addition of 300.

In this embodiment, the reinforcing member 320 comprises a steel core321 of a tubular construction which is oval in transverse cross-sectionso as to improve its resistance to bending, the oval being elliptical.Transition portions 325, 326 change the stress gradient between theflanges 323, 324 and the body portion 327 when a transverse bending loadis applied to the structural member 3. The thickness of the body portion327 of the casing 322 surrounding the core 321 is greater than thethickness of the core 321 to avoid rupture of the casing 322 when thecore 321 is deformed.

The reinforcing member 320 is manufactured as before and so will not bedescribed again in detail. It is sufficient to say that the tubular core321 is manufactured by extrusion or by any other convenient method suchas rolling and seam welding, the core 321 is then inserted in the cavity329 in the body portion 327 and the co-assembled parts are drawntogether through a die and cut to length and bent if required to producethe reinforcing member 320. As before, adhesive can be used if requiredbetween the core 321 and the wall of the cavity 29 in the body portion327.

In a further embodiment shown in FIG. 7, parts which are the same asthose shown in FIG. 6 carry the same reference while those which aresimilar carry the same reference with the further addition of 100. Thereinforcing member 420 differs from that described with reference toFIG. 6 in that while the core 421 is of a tubular construction that isoval in transverse cross-section, in this case the oval has parallelsides which are also parallel to the flanges 423, 424 and semi-circularend sections connected to the flanges 423, 424 through the transitionportions 425, 426.

In another embodiment shown in FIG. 8, parts which are the same as thoseshown in FIG. 6 carry the same reference while those which are similarcarry the same reference with the further addition of 200. Thereinforcing member 520 differs from that described with reference toFIG. 6 in that the core 521, while also being of a tubular construction,is trapezoidal in transverse cross-section, again to improve itsresistance to bending. This trapezoidal section is useful where thesection of the structural member 3 would not allow a symmetrical sectionof reinforcing member such as the reinforcing members 320, 420 shown inFIGS. 6 & 7.

The alternative form of “B” post 703 shown in FIG. 9 has a number ofsimilarities with the one shown in FIG. 2 so where appropriate the samereferences are used but with the addition of 700. The “B” post 703differs from those previously described in that it uses only onepressing 712 in combination with a reinforcing member 720. Thereinforcing member 720 comprises a steel core 721 encased in an aluminumcasing 722, the core 721 being non-planar in transverse cross-section toimprove its resistance to bending. The casing 722 has a body portion 727in which is located the core 721 and two longitudinally extendingflanges 723, 724 which are of uniform width and extend along the lengthof the body portion 727. The reinforcing member 720 is manufacturedexactly as before and so will not be described again in detail.

The reinforcing member 720 is fastened to the pressing 712 by placingthe reinforcing member 720 onto the second pressing 712 such that itsflanges 723, 724 rest upon and are aligned with the flanges 714, 716 ofthe pressing 712. The flanges 714, 716 of the pressing 712 and theflanges 723, 724 of the reinforcing member 720 are then secured togetherusing self-piercing rivets to form the structural member 703. Thisarrangement has the advantage of the use of less components andpotentially lower weight but can only be used when the reinforcingmember 720 can be produced in a shape required to form part of the outersurface of the structural member 703. This might be where the exposedsurface of the reinforcing member 720 is hidden by internal trimcomponents of the vehicle.

It will be appreciated that the invention is generally beneficial whenthe core is of a material that has a yield strength sufficiently high toprevent it being secured using self-piercing rivets but that a core ofultra high yield strength steel will provide the greatest strength. Inone proposed use of the invention it was calculated that an aluminumcasting or extrusion of approximately 10 to 12 mm wall thickness withthin rivetable joining flanges could be replaced by a structural memberaccording to the invention having a steel core 1.5 mm thick. It wasestimated that a 950 MPa yield heat-treated boron steel for the corewould provide four times the yield strength of the known aluminumsolution and provide a component approximately 80% of the weight of thesolely aluminum design and 50% of an all steel design. One example of anultra high strength steel suitable for use for the core has acomposition by mass of Carbon (C) 0.20%-0.25%, Silicon (Si) 0.15%-0.50%,Manganese (Mn) 1.00%-1.40%, Phosphorus (P)<0.030%, Sulfur (S)<0.025%,Chromium (Cr) 0.10%-0.35%, Molybdenum (Mo)<0.35%, Boron (B)0.0015%-0.0050%. Additionally minute amounts of Aluminum (Al) andTitanium (Ti) may have been added for manufacturing purposes.

Although the invention has been described above as including areinforcing member having a steel core and an aluminum casing it will beappreciated that other combinations could be produced with advantageouseffect. For example, a high yield strength steel core could be used toreinforce a casing made from another non-ferrous lightweight materialsuch as a magnesium alloy or a casing made from a low yield strengthsteel capable of being secured using self-piercing rivets. Similarly,although the invention has been described with reference to a “B” post,it will be appreciated that it could be used for other structuralmembers on a motor vehicle including movable components such as asuspension arm. Also, although the reinforcing member has been describedas joined to pressings to form the structural member, it may be joinedto panel members made by other means such as extrusion or casting.

1. A structural member for a motor vehicle, the structural member havingat least one panel member and a reinforcing member, the reinforcingmember comprising: an elongate core of a relatively high yield strengthmetal; and a casing of a relatively low yield strength metal having aconstant section profile with an internal wall defining a closed sectioncavity, the casing having a pair of spaced flanges connecting thereinforcing member to said at least one panel member wherein the core isencased within the casing and the core and the casing are bound togetherby co-drawing the core and the casing through a die.
 2. The structuralmember claimed in claim 1 wherein at least one of the casing, or theinternal wall of the core, is coated with an adhesive before co-drawing.3. The structural member claimed in claim 1 wherein the core is madefrom an ultra high strength steel.
 4. The structural member claimed inclaim 3 wherein the ultra high strength steel has a yield strength of atleast 500 MPa.
 5. The structural member as claimed in claim 1 whereinthe casing is made from lightweight non-ferrous material.
 6. Thestructural member claimed in claim 5 wherein the lightweight non-ferrousmaterial is an aluminum alloy.
 7. The structural member claimed in claim1 wherein the core is profiled flat strip, non-planar in cross-sectionand having two longitudinally extending edges.
 8. The structural memberclaimed in claim 7 wherein the material forming the longitudinal edgesis bent back on itself to form double-thickness end sections.
 9. Thestructural member claimed in claim 1 wherein the core is tubular. 10.The structural member claimed in claim 1 wherein the casing is joined tosaid at least one panel member by self-piercing rivets.
 11. A method ofmanufacturing a body structure for a motor vehicle including the stepsof: producing one or more reinforcing members by providing an elongatecore of a relatively high yield strength metal, a casing of a relativelylow yield strength metal, the casing having a constant section profilewith a pair of spaced flanges and an internal wall defining a closedsection cavity; placing the core within the cavity; co-drawing the coreand the casing through a die to bind the casing to the core; producingone or more panel members each having a pair of spaced flangescorresponding to the spaced flanges on the reinforcing member;connecting at least one reinforcing member and at least one panel memberat the spaced flanges to form structural members; providing at least oneor more body panels; attaching the structural members and the bodypanels together in a predetermined arrangement to produce the bodystructure.
 12. The method claimed in claim 11 wherein at least one ofthe casing or the internal wall of the core are coated with an adhesivebefore co-drawing.
 13. The method claimed in claim 11 wherein the coreis made from ultra high strength steel.
 14. The method claimed in claim13 wherein the ultra high strength steel has a yield strength of atleast 500 MPa.
 15. The method claimed in claim 11 wherein the casing ismade from lightweight non-ferrous material.
 16. The method claimed inclaim 15 wherein the lightweight non-ferrous material is an aluminumalloy.
 17. The method claimed in claim 11 wherein the core is profiledflat strip, being non-planar in cross-section and having twolongitudinally extending edges.
 18. The method claimed in claim 17wherein the material forming the longitudinal edges is bent back onitself to form double-thickness end sections.
 19. The method claimed inclaim 11 wherein the core is tubular.
 20. The method claimed in claim 11wherein each casing and the respective panel member are joined at theflanges by self-piercing rivets.